The present invention relates to the crystallisation of sugar by a process known as "transformation".
At present, the vast majority of crystalline sugar is produced by charging a hot, concentrated syrup into pans, drawing a vacuum over the pans and evaporating a proportion of the water from the syrup. A portion of the sugar then crystallises out and is separated, generally by a centrifuge. The mother liquor is then reboiled and recycled to produce another crop of sugar crystals. This process may be repeated a number of times until eventually there is produced a final molasses, from which the sugar cannot readily be crystallised; this final molasses is generally unsuitable for human use and usually only finds application as an animal feed or as a source of low grade carbohydrate. Although an extremely pure sugar is produced in the first crop, subsequent crops are of decreasing purity. Moreover, the process is very slow and complex. It has the further disadvantages that it can generally only be operated batch-wise and that it is ordinarily dependent upon the skill and judgement of the operator. A speedier and simpler process would be desirable, even if it is not capable of producing such pure sugar as is obtained in the first crop.
The process of sucrose transformation has been known in theory and practised to a limited extent for some considerable time. In this process, a sugar syrup is concentrated until it becomes supersaturated; aqueous sugar solutions can easily be supersaturated without nucleating simply by evaporative boiling. Nucleation is then induced by mechanical means, causing crystallisation of the sugar. Since sugar has a positive heat of crystallisation, the heat evolved during crystallisation will tend to evaporate water from the solution. Provided a suitable balance of temperature and concentration of the sugar syrup is achieved, essentially complete vaporisation of water can be attained to produce sugar having a very low moisture content. In order to prevent the formation of a solid mass of sugar crystals, it is necessary that the sugar syrup should be kept well agitated during crystallisation; this is normally achieved by stirring, e.g. using paddles, which may themselves provide the required nucleation. Although such a process works satisfactorily, it does not lend itself to continuous operation, and subsequent processing, such as milling and separation, is necessary to generate an acceptable marketable product. Furthermore, as crystallisation proceeds, the energy input necessary to break up the crystallising sugar mass also increases dramatically. The required energy input is so great that the plant necessary for commercial operation has to be massive, thus vitiating any economic advantages over conventional processes. A further disadvantage is the tendency of the crystallising sugar mass to clog apparatus. For these reasons, continuous processes tried have not been very successful.
In addition, the very substantial heat which is liberated during crystallisation will tend, if the sugar crystallises in bulk, to cause caramelisation, unless complicated means are adopted to reduce temperature.